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In the previous movie, you created an animated stroke in the layer's Alpha Channel. If I bring the Layer panel forward and scrub its timeline, I have painted an animated white stroke in the Alpha Channel. Because the layer is black and outside the layer is also black, I'm simply extending the Alpha Channel beyond the original source. And if the layer you are using is an interesting image, outside of that Alpha Channel might look quite strange, and we covered why that is in an earlier movie.
Now we need to finish off this effect so that the stroke wipes on the original layer. I have the Composition panel on the left. I will set it to 100% so you can see what's going on more clearly. I still have my Project panel. I'll just tidy up my workspace. I will select the layer and then select either F3 or open Effect>Effect Controls. And you will see that paint is an effect with just one parameter; Paint on Transparent. You will also notice that it does appear in the timeline, if you twirl up Paint and twirl it back down again.
But when you press U to see your keyframes, it doesn't show you that option automatically. So either in the timeline or in the Effect Controls panel, we need to turn on Paint on Transparent. We will turn that on and now at the beginning of the comp, the layer will be transparent. Again, my image is black and by extending the Alpha Channel, I am simply extending the black pixels. If you have a colorful image, you'll see the image wipe on, but the edge will not be clean.
So next, we need to apply an effect after the paint stroke that will retrieve the original Alpha Channel from the source and then apply it on top of the blobby strokes. The effect we are looking for is Effect> Channel>Set Matte, and that applies to Set Matte effect after the Paint effect. The defaults are actually just fine. What the Set Matte effect will do is take a matte from a layer and the default is to use the layer it's currently applied to.
The next parameter asks which channel should I use for a matte. Again, the default of Alpha Channel is correct. And by the way, the Set Matte effect is a very old effect. But once the Track Matte feature was added to After Effects, the Set Matte effect became less useful, but it is a way to use a matte from a different layer if you need to. You will notice in the Layer panel, it's also cleaned up the edges. So now the layer wipes on in the Alpha Channel as well.
Remember, in the View pop-up, you have a choice to see the Paint effect and that shows you the paint before the Set Matte has been applied. To see it after Set Matte, you will need to make sure Set Matte is enabled and to divide as a View pop-up, you will also see the Render checkbox. You want to make sure this is enabled as well. Otherwise, the effect is not actually rendered and all you see is the original image. Now, if you would like to practice this technique with any other layers, I will turn off the solo button and remind you that there are other layers in this comp.
We'll resize it with the triangle, notice if you start revealing from the center and then move along this line, the line eventually gets thicker. So if you are not using a pressure sensitive tablet, you may need to animate the diameter of the stroke. That way you can create a small stroke in the middle and then increase the diameter when the area you are trying to reveal becomes larger. If you have the exercise files, go ahead and preview the final composition again.
That might give you a few more ideas.
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